Conventionally, in the printing industry and electronics industry, a light source that can emit ultraviolet light is often used as a light source to cure, dry, melt, soften or reform an object to be treated (e.g., a protection film, an adhesive, painting, an ink, a photoresist, resin, and oriented film). In recent years, an LED element is becoming popular as the light source that emits light in an ultraviolet range (UV range). An ultraviolet light source unit that uses the LED element(s) to emit light in the UV range is therefore studied.
One example of a configuration that includes the light source unit having the LED element(s) together with an ink jet head of an ink jet printer is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-358769 (Patent Literature 1; will be mentioned below).
FIG. 15 of the accompanying drawings shows this configuration.
An ink jet printer 20 includes an ink jet head 21 to inject an ink to a print medium M such as paper, and ultraviolet light source units 22 located on one side (or both sides) of the ink jet head 21. The ink jet head 21 and the ultraviolet light source units 22 are situated above the print medium M by a predetermined distance, and supported from a guide rail 23 such that the ink jet head and the ultraviolet light source units can move (scan) in a transverse direction X relative to the print medium M.
A UV ink droplet injected from the ink jet head 21 and adhering to the surface of the print medium M is cured by the ultraviolet light emitted from the light source unit(s) 22. Accordingly, the UV ink is successively cured (fixed) on the surface of the print medium M in the scanning direction X of the ink jet head 21.
After the UV ink droplet is cured, the print medium M is moved a prescribed distance in the length direction Y, and the above-mentioned printing is repeated. In this manner, a picture (drawing, painting) or a character (letter) is made on the surface of the print medium M.
In the above-mentioned Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2004-358769), the ultraviolet light source unit provided in the ink jet printer has LED elements that are arranged zigzag (staggered). FIG. 16 of the accompanying drawings shows the arrangement of the LED elements. A light irradiation surface of the ultraviolet light source unit 22 has a substrate 222, on which the LED elements 221 are disposed. The LED elements 221 are situated on the substrate 222 zigzag in the longitudinal direction and the transverse direction. With such zigzag arrangement of the LED elements, the ultraviolet light source unit 22 can emit the ultraviolet light uniformly, without gaps, along the traveling path of the ultraviolet light source unit 22 when the ultraviolet light source unit 22 is moved.
If the LED elements 221 are serially connected on the substrate 222, and one of the LED elements 221 is disconnected, then all the remaining LED elements 221 which are serially connected to that LED element are also disconnected. Thus, the light source unit is brought into the unlit condition.
To deal with it, inventors devised a different wiring arrangement as shown in FIG. 17. In FIG. 17, a plurality of parallel linear or narrow strip-like wirings 12 are arranged in the same direction on the substrate 11. A plurality of LED elements 13 are connected to each of the strip-like wirings 12 by soldering. When the substrate 11 is viewed as a whole, the LED elements 13 are arranged zigzag. Each of the LED elements 13 on each strip-like wiring 12 has an upper face electrode 14, and a wire 15 extending from the upper face electrode 14 is electrically connected to a region 16 of a next strip-like wiring 12 between each two adjacent LED elements 13 on that strip-like wiring 12 by wire bonding.
With such configuration, each strip-like wiring 12 becomes a common electrode for the LED elements 13 disposed on that strip-like wiring 12. Thus, the LED elements 13 on each strip-like wiring 12 are electrically connected in parallel to each other.
Such wiring configuration brings about an advantage that even when one of the wires 15 of the LED elements 13 is disconnected, other LED elements are not unlit correspondingly.
It was found, however, that when the above-described configuration was actually tried, wire connection (wire bond) between the LED elements and the strip-like wirings suffered from poor connection.
This shortcoming will be described with reference to FIGS. 18(A) and 18(B) of the accompanying drawings.
As described above, the LED elements 13 are joined to the strip-like wirings 12 by soldering, and the LED elements 13 on one strip-like wiring are connected to the neighboring strip-like wiring 12 at the regions 16 between the LED elements 13 on that strip-like wiring 12 by the wires 15. When the LED elements 13 on one strip-like wiring are soldered to the strip-like wiring 12 on the neighboring strip-like wiring, the solder 17 and fluxes contained in the solder 17 are melted, and flow from the lower faces of the LED elements 13. It is difficult to control (regulate) the outflow of the solder 17. Also, the LED elements 13 are arranged at intervals as small as possible from the viewpoint of reducing the installation areas of the LED elements 13. Thus, the solder 17 that is used to connect one LED element 13 often becomes continuous to the spilled solder from an adjacent LED element 13.
As shown in FIGS. 18(A) and 18(B), if the solder 17 of one LED element 13 becomes continuous to the solder 17 of an adjacent LED element 13 on one strip-like wiring, there is the solder 17 on the region 16 between these two LED elements 13. The wire 15 extending from the upper face electrode 14 of the LED element 13 on one strip-like wiring 12 is bonded to such region 16 on an adjacent strip-like wiring by wire bonding. However, because the solder 17 is present on the region 16, the wire 15 cannot be properly bonded to the region 16. This results in poor wire connection.
When the above-described ink jet printer should have a faster printing speed to increase the processing speed, the ink jet and the light source unit need to move quickly in the transverse direction X. However, when the light source unit is moved quickly in the X-direction, an amount of ultraviolet light irradiation that is directed to the UV ink droplet injected from the ink jet head per unit time becomes smaller. Accordingly, the ink is not cured sufficiently. In order to completely cure the ink on one hand and increase the processing speed on the other hand, it is necessary to increase an amount of ultraviolet light irradiation. To achieve this, the LED elements need to be disposed at a high density in a prescribed area on the substrate.
In order to realize the high density arrangement of the LED elements in the light source unit, the inventors studied the possible high density arrangement of the LED elements and the strip-like wirings on the substrate, i.e., the inventors studied the above-described configuration as shown in FIG. 17.
As shown in FIG. 19, which is a cross-sectional view taken along the line A-A in FIG. 17, the LED element 13 has the electrode 14 on its upper face and the electrode 18 on its lower face. The electrode 18 attached to the lower face is connected to the strip-like wiring 12 by soldering or the like. The electrode 14 attached to the upper face is connected to the adjacent strip-like wiring 12 between the LED elements 13 by the wire 15.
The strip-like wirings 12 should be insulated from each other such that the electric circuits formed by the respective strip-like wirings 12 are insulated from each other. Thus, the strip-like wirings 12 are spaced from each other at prescribed intervals for insulation (gaps for insulation). The heat sink HS is provided in contact with the lower face of the substrate 11. Heat of the substrate is radiated from the heat sink HS with the cooling air from a cooling fan. Thus, the substrate 11 is cooled. With such configuration, the heat generated from the LED elements 13 is radiated through the lower face electrode 18, the strip-like wiring 12, the substrate 11 and the heat sink HS in this order (heat radiation route from the LED lower face electrode to the heat sink).
When the configuration of FIG. 17 is employed, the strip-like wirings 12 should have the prescribed insulation gaps (should be spaced from each other). In the end, this becomes an obstacle, i.e., this makes it difficult to arrange the LED elements 13 on the strip-like wirings 12 at a high density in the X-direction perpendicular to the longitudinal direction of the strip-like wirings 12.
To deal with this, the width of each strip-like wiring 12 may be made smaller than the width of the LED element 13. Such strip-like wirings 12 may be arranged in parallel, and the LED elements 13 may be disposed on the strip-like wirings 12 to increase the arrangement density (installation density) of the LED elements 13 in the X-direction.
FIG. 20 illustrates the above-described structure. In this drawing, each of the strip-like wirings 12 on the substrate 11 has a smaller width than each LED element 13. A plurality of LED elements 13 are arranged on each wiring 12.
Because the width of each of the strip-like wirings 12 is reduced, the insulation gap between each two adjacent wirings 12 is reduced, as compared to the configuration shown in FIG. 17, even if the wirings 12 are spaced from each other at the prescribed insulation gaps. Accordingly, the LED elements 13 disposed on the wirings 12 may be arranged at a higher density in the X-direction, which is perpendicular to the longitudinal direction of the wirings 12.
When the above-described configuration is employed, however, the width L1 of the strip-like wiring 12 on the substrate 11 becomes smaller than the width of the LED element 13, as shown in FIG. 21 which is the cross-sectional view taken along the line A-A in FIG. 20. If the installation density should be further reduced in the X-direction of FIG. 20, then the width L1 would become smaller than the width L2 of the lower face electrode 18 of the LED element 13 (L1<L2). This makes the heat radiation route from the LED element 13 to the heat sink HS narrow(er) at the strip-like wiring 12, and deteriorates the heat radiation capability. As a result, a sufficient cooling effect is not given to the LED elements, and the luminous efficacy (light emission efficiency) of the LED elements drops.